Convert principals into a list if needed

If the module is used together with pyramid_multiauth and a Paste-Deploy configuration file is used to configure the module then the constructor gets the list of principals in form of string from pyramid_multiauth and it needs to be converted into a list.

Without this the principals string will be appended to the list of principals character by character
Merge pull request #2 from janakj/patch-1

Convert principals into a list if needed